It's interesting that prior to the industrial revolution there are still some periods where it seems like innovations arrived relatively fast, and others where it was comparatively slow. E.g. a lot more entries are in the 500 BCE - 200 BCE period than the 200 - 500 range.
Although the idea of a "Dark Age" is mostly debunked these days, the slow unraveling of the Western Roman Empire led to a real and sustained change in material conditions. Notably, population density and urbanization both decreased, along with the labor specialization that accompanies them. I'd expect most 'inventions' to happen when and where people have the most hands on time to make them! (I can't really speak to Indian and Chinese civilizations, but they have also had integration and disintegration periods)